The invention relates to a fluid flow device for the passage of liquids or moist gases, having a structure which substantially prevents freezing (and thus blockage) of the duct. Such devices are used in vehicle construction, particularly in modern fuel cell power systems, to broaden the operating temperature range of such power systems. As in conventional internal combustion engines, a particularly critical factor for fuel cell systems is the cold start phase, in which no process heat is as yet available to the main units for thawing purposes.
German patent document DE 3129836 C1 discloses a method of introducing a capillary structure (such as, for example, a stainless steel network) onto the inside of a long, internally smooth pipe, in order to distribute liquid evenly over the internal circumference of the pipe by capillary forces. By making use of the capillary action, the freezing point may be kept somewhat lower, thereby delaying freezing up of the ducts.
German patent document DE 10063254 A1 also discloses a device for keeping ducts open by distributing liquid water via capillary forces. To this end, a porous coating (for example, aluminosilicate or zeolite) is applied to the inside of the pipe; the coating absorbs water in disperse form at lower temperatures and releases the water again at least in part at higher temperatures. Such coatings, which absorb water in disperse form at low temperatures in order to prevent the formation of water droplets and the coalescence of water droplets into larger water quantities, have only a limited capacity for liquid absorption, however.
Against this background, one object of the present invention is therefore to provide a device for passing a flow of liquids or moist gases which largely prevents freezing and thus blocking of the duct.
Another object of the invention is to provide a fluid flow device which straightforwardly enables trouble-free operation to be established at sub-zero temperatures, without allowing deposited water (and water accumulated into larger quantities) to prevent operation of the fuel cell power systems due to freezing, or even to damaging the device as a result of frost.
These and other objects and advantages are achieved by the flow passage structure according to the invention, in which a nonwoven provided on at least part of the internal wall flow passages or flow chambers absorbs liquid in disperse form at low temperatures. Thus, the invention is based on the recognition that capillary, spatial structures exist which absorb liquids in disperse form at relatively low temperatures. It thereby prevents (or at least impedes) the formation of water droplets, for example, and the coalescence of larger liquid accumulations, which may then freeze to form a coherent, block-forming structure. Even if freezing were to occur, ice formation would be restricted by the nonwoven to just local, individual crystal nuclei, so preventing frost damage.
In a further advantageous embodiment of the flow passage device according to the invention, the nonwoven is made of stainless steel. In this way, probable contamination caused by unintentional corrosion or even material detachment, which could lead to a malfunction of or even damage to the fuel cell power system, is largely avoided even with large temperature bandwidths.
In a further advantageous embodiment, the nonwoven is of multi-ply construction. In this way, functional differentiation between the different plies of the nonwoven is possible.
The multi-ply nature of the nonwoven is as follows:                The nonwoven with a supporting function is directed with a coarse mesh structure towards the wall-side duct surface. This ply has the function of keeping the complete nonwoven composite in shape statically in the duct, while providing the stability necessary for fitting during the insertion process.        The nonwoven with a capillary action is directed with a fine mesh structure towards the middle of the duct, and is thus in direct contact with the liquid or the moist gases of the fuel cell process. With the static support provided by the nonwoven structure with the coarse mesh structure, the functional purpose of the nonwoven ply with the fine mesh structure is to make the pores of the surface structure so small that condensing liquid is absorbed in a virtually capillary manner. Accordingly, no larger liquid accumulations, which could freeze and lead to blocking of the fuel cell system, are able to remain on the surface of the nonwoven.        
In a further advantageous embodiment of the nonwoven plies, the nonwoven is coated with a fine mesh structure with special chemicals to prevent condensate formation. Aluminum silicates and polymers with acid or alkali residues have proven to be practical coatings. Such hydrophilic coatings, with their chemical affinity for water, also reduce the surface tension of the liquid, so in turn preventing relatively large liquid accumulations which could freeze and lead to blockage of the fuel cell system.
If, at extremely low temperatures, neither the previously described physical capillary action nor the chemical hydrophilic action of the nonwoven coating can prevent freezing, a heating conductor is built in to keep the duct and the nonwoven fabric clear. Such a heating conductor with electrical heating wires is advantageously incorporated into the supporting, coarse mesh structure of the nonwoven ply on the wall side. In a further embodiment, the nonwoven ply on the wall side may itself be of two-ply construction and the heating conductor may be pressed or adhesively bonded between the two plies of the wall-side nonwoven ply. The heating conductor is only in operation until the process heat from the fuel cell power system prevents freezing, so as not to load the vehicle electrics unnecessarily.
In an advantageous method of filling the closed duct with the nonwoven or nonwoven composite, the nonwoven is inserted into the closed duct in a rolled-up state, and is unrolled only when it is finally located at the point in the duct necessary for functioning. The rolled-up nonwoven may be drawn through, for example, using the heating element terminal wires previously drawn through the duct.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.